Swift里字符串（六）Shared strings

Shared strings do not have tail-allocated storage, but can provide access upon query to contiguous UTF-8 code units. Lazily-bridged NSStrings capable of providing access to contiguous ASCII/UTF-8 set the ObjC bit. Accessing shared string's pointer should always be behind a resilience barrier, permitting future evolution.

字符串数据没有从内部_object处起始，但是可以获取连续的 utf-8 code unit。
对应的类是__SharedStringStorage。

内存布局



在 OC runtime下，是 NSString 子类



子类化 NSString

#if _runtime(_ObjC)

  @objc(length)
  final internal var length: Int {
    @_effects(readonly) get {
      return asString.utf16.count // UTF16View special-cases ASCII for us.
    }
  }

  @objc(characterAtIndex:)
  @_effects(readonly)
  final internal func character(at offset: Int) -> UInt16 {
    let str = asString
    return str.utf16[str._toUTF16Index(offset)]
  }

  @objc(getCharacters:range:)
  @_effects(releasenone)
  final internal func getCharacters(
    _ buffer: UnsafeMutablePointer<UInt16>, range aRange: _SwiftNSRange
  ) {
    _getCharacters(buffer, aRange)
  }